Lignocellulosic materials represent a major renewable energy and carbon source. Unfortunately, the chemical and physical structures of the vast majority of lignocellulosic materials hamper efficient utilization of this material for many purposes. This invention relates to a lignocellulose treatment process using low cost chemical feedstocks. The process economically converts lignocellulosics to a usable form.
The structure of lignocellulose fibers varies widely depending on the source. However, most lignocellulosic materials are complex structures comprising cellulose, hemicellulose and lignin as major constituents.
Cellulose itself is a variable polysaccharide, commonly a linear polymer of D-anhydroglucopyranose units connected with .beta.-1-4-glucosidic bonds.
Hemicellulose exhibits wide variation in composition; however, hemicellulose commonly comprises polymers or heteropolymers of galactose, mannose, xylose, arabinose, and other sugars, together with uronic acids derived from those sugars. Hemicellulose generally exhibits a degree of polymerization of up to about 200 sugar moieties.
Lignin is a cross-linked three-dimensional polymer of aromatic alcohols, typically coniferyl alcohol units or a combination of coniferyl and syringyl alcohols.
Mammals do not produce the enzymes necessary to hydrolyze the .beta.-1-4-glucosidic bonds of cellulose. Such enzymes, however, are produced by a number of microorganisms, and such microorganisms are found, e.g., in a symbiotic relationship in the stomachs of ruminants to facilitate the digestion of lignocellulosic materials.
Microbial or enzymatic degradation of lignocellulose is often relatively ineffective and/or unacceptably slow for many agricultural and commercial purposes. It has been found that the degree of crystallinity of the lignocellulose material and the presence of lignin and hemicellulose are significant factors in impeding the degradation or decomposition of lignocellulose. Indeed, the crystalline regions of cellulose and regions of lignocellulosic materials containing a substantial amount of lignin are notoriously resistant to biological degradation.
For these reasons, the abundant lignocellulosic agricultural residues, including straw, chaff, corn stover, and the like, are waste products that presently have little or no commercial value. Moreover, these materials decompose slowly in the environment and often present waste disposal problems.
Various prior art processes have been developed to reduce the crystallinity of lignocellulosic materials and/or to remove lignin from these materials. One of the most effective is an alkaline hydrogen peroxide pretreatment that disrupts the crystallinity of the cellulose and solubilizes the lignin. See, e.g., J. Gould, Alkaline Peroxide Delignification of Agricultural Residues to Enhance Enzymatic Saccharification, Biotechnology and Bioengineering 26:46-52 (1984); Wei et al., Effect of Hydrogen Peroxide Pretreatment on the Structural Features and the Enzymatic Hydrolysis of Rice Straw, Biotechnology and Bioengineering, 27:1418-1426 (1985); and J. Gould, U.S. Pat. No. 4,649,113. By treating lignocellulosic materials (particularly those from non-woody plants) with dilute hydrogen peroxide at a pH of about 11.5, lignocellulosic materials may be converted into material that is easily digested by ruminant animals. See M. Kerley, et al., Alkaline Hydrogen Peroxide Treatment Unlocks Energy and Agricultural By-Products, Science 230: 820-822 (1985). In this study, lambs fed on alkaline hydrogen peroxide-treated wheat straw gained weight at a rate comparable to that of lambs consuming a diet composed predominantly of corn.
Although humans lack the ability to digest cellulose, cellulose is nevertheless desirable in human nutrition as a source of dietary fiber. It has been demonstrated that cellulose fiber from straw or other sources that has been processed with alkaline hydrogen peroxide can be milled into a white powder and used as a replacement for up to 40% of the flour in baked goods, without any perceptible deleterious effect on the resulting product. Indeed, bread products made with wheat flour and 30% cellulose fiber form a gluten network having a higher tensile strength than that of bread made with flour alone. The fiber-containing bread also exhibits an increased volume.
Use of such lignocellulose fibers as dietary fiber can simultaneously increase the non-nutritive dietary fiber content and decrease the caloric value of the products made with the fiber. In light of public health concerns related to inadequate consumption of dietary fiber and excess consumption of calories, both of these factors are believed to be highly desirable.
One of the major costs involved in the alkaline hydrogen peroxide process for treatment of lignocellulosic materials is the cost of the hydrogen peroxide. Moreover, a significant concern related to the use of that process is the disposal of the solubilized hemicellulose and lignin in the effluent stream of the process. Both of these factors could have a significant impact on the economics and feasibility of commercializing this lignocellulose treatment process.